In recent years, there are growing demands for hot-rolled steel sheets for automobiles having enhanced strength and reduced weight from the viewpoint of improvement in safety of automobiles and reduction in fuel consumption, which leads to environmental conservation. Among the automobile parts, frame-related parts and arm-related parts, which are called an underbody system, occupy a large portion of the entire weight of the vehicle. Thus, the entire weight of the vehicle can be reduced by enhancing the strength of materials used for these parts, and reducing the thickness of these parts. Further, press forming is widely used for shaping materials into the underbody system. Thus, in order to prevent these materials from cracking during the press forming, these materials are required to have a high bending workability. For this reason, high-strength steel sheets are widely used. In particular, hot-rolled steel sheets are mainly used because of their price advantages. Yet further, for reinforcing members or underfloor members, in particular, for slide rails for seats or other small members subjected to the bending working, cold-rolled steel sheets or zinc-plated steel sheets are mainly used to reduce the thickness thereof and reduce the weight thereof through use of the high-strength steel sheets.
Of the steels described above, there are known a low-yield-ratio DP steel sheet containing a ferrite phase and a martensite phase, and a TRIP steel sheet containing a ferrite phase and a (retained) austenite phase, as a high-strength steel sheet having increased strength, improved workability and improved formability. However, although exhibiting increased strength and excellent workability and ductility, these steel sheets do not have excellent hole expandability, in other words, stretch-flange formability or bending workability. Thus, in general, although ductility is slightly inferior, bainite-based steel sheets are used for structural parts such as underbody components that are required to have the stretch-flange formability.
One of the reasons that a composite-structure steel sheet including the ferrite phase and the martensite phase (hereinafter, also referred to as “DP steel sheet”) has lower stretch-flange formability is considered to be that, since this steel sheet is a composite formed by the soft ferrite phase and the hard martensite phase, stress concentrates on a boundary portion between both phases during the hole-expansion working, and the steel sheet cannot follow its deformation, whereby this boundary portion is likely to become a start point of breakage.
To solve the problems described above, several steel sheets are proposed on the basis of the DP steel sheet with the aim of achieving both the mechanical strength property and the bending workability or hole-expandability (workability). For example, as a technique for stress relaxation using fine dispersed particles, Patent Document 1 discloses a composite-structure steel sheet including a ferrite phase and a martensite phase (DP steel sheet) in which fine Cu precipitates or solid solutions are dispersed. In this technique disclosed in Patent Document 1, it is found that the bending workability can be significantly effectively improved without deteriorating the workability, by using Cu precipitates having a particle size of 2 nm or less and formed by Cu in solid solution or Cu alone, and on the basis of the findings, a composition ratio of contained components is defined.
As a technique for stress relaxation by reducing the difference in strength in composite phases, for example, Patent Document 2 discloses a technique relating to a bainite steel, in which the difference in hardness between ferrite and bainite is reduced by minimizing C as much as possible to make the bainite structure become the primary phase, and adjusting the ferrite structure, which has been subjected to solid solution strengthening or precipitation hardening, so as to have an appropriate volume ratio, and further, generation of coarsened carbides is eliminated.
Patent Document 3 discloses a technique of obtaining a high-strength steel sheet exhibiting excellent bending workability, by defining the size and the number of oxide-based inclusions on the assumption that the oxide-based inclusions cause cracking during the bending working.
Further, Patent Documents 4 and 5 disclose a technique of obtaining a high-strength steel sheet exhibiting excellent stretch-flange formability and fatigue characteristics, by reducing the size of elongated MnS-based inclusions existing in the steel and deteriorating the fatigue characteristics and the stretch-flange formability (hole expandability), to be fine spherical inclusions, which are less likely to be a starting point of the occurrence of cracking, and dispersing the fine spherical inclusions in the steel.